Field of the Invention
The invention relates generally to secure documents and more particularly to systems and methods for encoding information therein.
Description of the Related Art
Watermarking is a technique well-known in the art of secure documents which derive their value substantially from the ability of users to rely confidently upon their authenticity and, in this connection, their resistance to counterfeiting. Examples of such documents include tangible documents such as banknotes, passports, driver's licenses, identification cards, and lottery tickets, and also intangible documents such as electronic documents.
Watermarking may be employed in steganography by incorporating into documents hidden information which is both unintelligible to an observer and whose presence in the document is not apparent absent knowledge of the means of its incorporation. A common method of this type is the inclusion in a secure document of a host image, e.g. a picture of a famous landmark, or a portrait of a notable person, which is itself readily perceivable by an observer, but within which a hidden image is encoded and is neither perceivable by an observer nor whose presence is either indicated or suggested by the host image or any other aspect of the document. The intention of such methods is that nothing in the document leads anyone apart from the document producer and the document authenticator to suspect the presence of hidden content, thereby rendering the document less susceptible to attempted counterfeiting or infiltration.
With the advent of electronic commerce, secure documents now take digital form in addition to the traditional, tangible form of banknotes and the like, and so digital watermarking is now widely practiced. Moreover, the production of tangible secure documents now typically proceeds by first developing a digital image of the document which is then used to produce a tangible document using known printing methods. The effective use of digital watermarking in the production of tangible secure documents depends in part, however, on the specific printing method employed, and in particular any limitations associated with that printing method. Unless such limitations are taken into account, it is possible that any hidden information, such as a digital watermark, encoded in a digital image used to produce the tangible document may be undesirably altered when printed and thereby rendered ineffective as a security feature.
In this connection, it is noted that, with the notable exception of laser marking and dye diffusion printing, both analog and digital printing methods involve the deposit of a pigment on a substrate without controlling the obtained optical density within the points or the lines defining the printed graphic. The perception by the unaided eye of a shade of grey in a zone perceived as a geometric point is typically achieved by controlling the proportion of that area covered by the pigment. This type of printing is often called ‘binary halftoning’. The common use of binary halftoning, however, presents problems when used to incorporate a digital watermark as a hidden image in a host image. Specifically, the translation of digital steganography in the field of binary halftone printing becomes difficult as the range of grey values reduces to two, i.e. the absence of or the presence of a point of impression. There exist at least two known methods of addressing this limitation.
In a first method, as exemplified in WIPO International Publication No. WO 99/53428 by Digimark Corporation [the “Digimark method”], a coarse partition is defined which is significantly larger than the printing points used in the impression associated with the definition domain of the host image. The mean value of the host image field is computed for each coarse partition subset, or constituent region, and these mean values become the entry values in a matrix associated with the original image. Digital watermarking algorithms are then applied to this coarse partition matrix which allows real valued entries instead of only binary values. The process of incorporating the watermark into the host image involves the slight thickening, thinning, or deviation of the lines or points contained within each constituent region in order to achieve a specific increase or decrease in the average optical density associated with that region. The result is an alteration of the average values of the original image in these regions, whereby the steganographic information is embedded. In general, this method is suitable for artwork texture having an almost uniform average optical density.
In a second method, as exemplified in U.S. Pat. No. 5,315,098 to Tow [the “Tow” method], the unique, uncontrolled impression point shapes typically employed in binary halftoning are replaced with a predetermined number of specific shapes each having a predetermined angular anisotropy, e.g. ellipses, wherein the rotation of the major axis of any such ellipse with reference to a predetermined reference axis may be selected within certain limits whereby a predetermined number of angular ranges may be discriminated. By this technique, the informational space associated with the basic element of binary printing may be broadened. A binary halftone image may be represented using such anisotropic points wherein a finite number N (e.g. two) of different specific angular ranges are permitted. Detection of the anisotropic points of any particular range may be performed by linear filtering with N such convolution operations being necessary.
The two above-described methods may be distinguished by a comparison of the initial resolution of the host image and the effective resolution of the watermark. In the first method, the digital watermark information manifests at a resolution significantly coarser than the original host image. This method may be characterized as encoding in the low frequency region of the Fourier spectrum associated with the host image. The inconspicuous character of the watermark is achieved by altering the average values of the optical reflectance of the partition subsets within the threshold of discrimination of human vision of grey levels. In the second method, the selective shaping of the impression points implies an increase of the host image effective resolution, but wherein again the spectral footprint of the anisotropic impression points remains confined around the origin of the spectral plane. In this case, the inconspicuous character of the watermark is achieved by the selective rotation of the anisotropic impression points outside of the angular limit of acuity of human vision.
While the above-described methods provide certain advantages for encoding hidden images in secure documents, they suffer from certain shortcomings.
The Digimark method, for example, relies substantially on precise control over the alteration in luminosity of any partition constituent region, and is vulnerable to unintended non-uniform changes in the luminosity of the host image. In other words, since the hidden information in the Digimark method is stored in very subtle variations in local luminosity, any unintended non-uniform changes in the luminosity of the host image would corrupt the hidden information, rendering the method unsuitable where robust storage of the hidden information is required. Moreover, since the Digimark method uses subtle variations in local luminosity to encode the hidden information, this technique is removed from other practical uses, e.g. to create in the host image a visible image by selectively altering the thickness of the constituent lines. The Digimark method therefore inefficiently employs for encoding of hidden information a property of the host image which is also useful for constructing a visible image.
In the Tow method, on the other hand, binary information is stored in the relative rotation of a plurality of binary halftone impression points which are purposefully anisotropic. Each such impression point is required to contain a bit of the hidden information. The method, therefore, relies substantially on a considerable degree of precision in both producing such anisotropic impression points, but also the detecting of their relative rotation during decoding, and is therefore highly sensitive to any artefact which might affect the produced or detected rotation of each impression point.
There remains, therefore, a need for novel and effective methods of encoding hidden images in security documents for combating counterfeiting and infiltration.